A parametric loudspeaker is a sound emission system that directly generates ultrasonic frequencies into a medium such as air. The parametric array in air results from the introduction of sufficiently intense, audio modulated ultrasonic signals into an air column. Self demodulation, or down-conversion, occurs along the air column resulting in an audible acoustic signal. This process occurs because of the known physical principle that when two sound waves with different frequencies are radiated simultaneously in the same medium, a sound wave having a wave form including the sum and difference of the two frequencies is produced by the non-linear interaction (parametric interaction) of the two sound waves.
For example, if the two original sound waves are ultrasonic waves and the difference between them is selected to be an audio frequency, an audible sound is generated by the parametric interaction. The result is a highly directional loudspeaker that is effectively a virtual end fired array. Historically these devices have not been able to achieve high performance for multiple reasons, much of which can be attributed to transducer performance. In the prior art, devices are disclosed that use piezoelectric bimorph devices which are also known as piezoelectric benders. The prior art systems have used clusters of piezoelectric bimorphs that number anywhere from 500 to over 1400 bimorph units. The large number of bimorphs is due to the very high ultrasonic outputs required for a parametric loudspeaker. The output performance from these bimorph devices has not been adequate in prior art systems.
An example of the prior art is described in the article, “The audio spotlight: An application of nonlinear interaction of sound waves to a new type of loudspeaker design.”, by Yoneyama and Fujimoto in the Journal of the Acoustical Society of America, Volume 73, 1983, which is incorporated herein by reference. Their use of an array of 547 piezo bimorph type transducers typifies previous and subsequent prior art parametric loudspeakers.
As with other prior art parametric loudspeakers, Yoneyama teaches placing the primary carrier frequency or carrier signal at the transducer's resonance frequency which is the frequency of maximum amplitude for a single transducer. This is the region of highest amplitude and has been presumed to provide the best performance for an array of transducers. Further, Yoneyama also teaches the mounting of the multiple transducers all in the same plane. However, it is believed that such prior art arrays all suffered from the disproportionate loss of sound pressure hi level (SPL) with increasing numbers of transducers
Accordingly, it would be an improvement over the state of the art to provide a new apparatus and method for a parametric loudspeaker that uses multiple transducer devices and operates with improved phase matching and provides increased output.